Mental illness afflicts nearly ten percent of the general population both in the United States and in the rest of the world. Bipolar (manic depressive) disorders occur in one to two percent of the population and are the sixth leading cause of disability (Coryell et al., Am. J. Psychiatry 150:720-727 (1993); Lopez, A. D., and Murray, C. C., Nat. Med. 4:1241-1243 (1998); Hyman, S. E., Am. J. Geriatr. Psychiatry 9:330-339 (2001)). A problem facing the medical community is misdiagnosis of a bipolar disorder. Misdiagnosed patients receive an average of 3.5 misdiagnoses and consult four physicians before receiving an accurate diagnosis (“Living with bipolar disorder: How far have we really come?” National Depressive and Manic-Depressive Association, Chicago, Ill. (2001)).
The current diagnostic method for a bipolar disorder (bipolar disorder I and II) involves a series of clinical interviews and examination using the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), the main diagnostic reference of Mental Health professionals in the United States, which is now in its fourth edition. Significant controversy exists about the validity of this manual, which limits the accuracy of clinical diagnosis (Torrey et al, “Surviving Manic Depression”, Basic Books, New York (2002)). In addition, attempts are underway to identify the genes underlying these illnesses and thereby develop diagnostic markers. However, their identification and possible use as diagnostic markers are years away (Bradbury, J., Lancet 357:1596 (2001)).
The lifetime risk for unipolar disorder (major depressive disorder) is 10% to 25% for women and from 5% to 12% for men. At any point in time, 5% to 9% of women and 2% to 3% of men suffer from this disorder. Prevalence is unrelated to ethnicity, education, income, or marital status.
Like bipolar disorders, unipolar disorder is also currently diagnosed using the DSM-IV. By definition, unipolar disorder and bipolar disorders are distinct conditions. Unipolar disorder is diagnosed when there has never been a manic episode and at least five of the following symptoms have been present during the same 2 week depressed period:                Abnormal depressed mood.        Abnormal loss of all interest and pleasure.        Appetite or weight disturbance, either:                    Abnormal weight loss (when not dieting) or decrease in appetite.            Abnormal weight gain or increase in appetite.                        Sleep disturbance, either abnormal insomnia or abnormal hypersomnia.        Activity disturbance, either abnormal agitation or abnormal slowing (observable by others).        Abnormal fatigue or loss of energy.        Abnormal self-reproach or inappropriate guilt.        Abnormal poor concentration or indecisiveness.        Abnormal morbid thoughts of death or suicide.        
There is evidence that unipolar disorder is, in part, a genetic disorder. Therefore, as with bipolar disorders, attempts are underway to identify the genes underlying unipolar disorder and thereby develop diagnostic markers. However, this has yet to be achieved.
In virtually every animal cell, the concentration of Na+ in the cell (˜12 mM) is lower than the concentration of Na+ in the surrounding medium (˜145 mM), and the concentration of K+ in the cell (˜140 mM) is higher than the concentration of K+ in the surrounding medium (˜4 mM). This imbalance is established and maintained by an active transport system in the plasma membrane. The transporter enzyme Na+K+ ATPase, also known as the sodium pump, couples breakdown of ATP to the simultaneous movement of both Na+ and K+ against their electrochemical gradients. For each molecule of ATP hydrolyzed to ADP and Pi, the Na+K+ ATPase transports two K+ ions inward and three Na+ ions outward across the plasma membrane.
The Na+K+ ATPase is an integral protein with two subunits (Mr ˜50,000 and ˜110,000), both of which span the membrane. A proposed mechanism by which ATP hydrolysis is coupled to ion transport involves the Na+K+ ATPase cycling between two forms, a phosphorylated form with high affinity for K+ and low affinity for Na+, and a dephosphorylated form with high affinity for Na+ and low affinity for K+. The conversion of ATP to ADP and Pi occurs in two steps catalyzed by the enzyme.
In addition to the Na+K+ ATPase, the plasma membrane also contains channel proteins that allow the principal cellular ions (Na+, K+, Ca2+, and Cl−) to move through them at different rates down their concentration gradients. Ion concentration gradients generated by pumps and selective movement of ions through channels constitutes the principal mechanism by which a difference in voltage, or electric potential, is generated across the plasma membrane. However, because the plasma membranes of animal cells contain many open K+ channels, and relatively few open Na+, Ca2+, and Cl− channels, the membrane potential in animal cells depends largely on open K+ channels. As a result, the major ionic movement across the plasma membrane is that of K+ from the inside outward, powered by the K+ concentration gradient, leaving an excess of negative charge on the inside and creating an excess of positive charge on the outside.
The magnitude of this membrane potential generally is −50 mV to −70 mV (with the inside of the cell negative relative to the outside), which is characteristic of most animal cells and essential to the conduction of action potentials in neurons. As noted earlier, the K+ concentration gradient that drives the flow of K+ ions through open K+ channels is generated by the Na+K+ ATPase. The central role of the Na+K+ ATPase is reflected in the energy invested in this reaction: about 25% of the total energy consumption of a human at rest.
The steroid derivative ouabain is a potent and specific inhibitor of the Na+K+ ATPase. Ouabain and another steroid derivative, digitoxigenin, are the active ingredients of digitalis, which has long been used to treat congestive heart failure. Inhibition of the Na+K+ ATPase by digitalis leads to an increased Na+ concentration in cells, activating a Na+ Ca2+ antiporter in cardiac muscle. The increased influx of Ca2+ through this antiporter produces elevated cytosolic Ca2+, which strengthens the contractions of heart muscle.
The Na+K+ ATPase has also been investigated for its possible involvement in bipolar disorder pathophysiology (El-Mallakh et al, Biol. Phychiatry, 537:235-244 (1995)). However, this has been an unsettled and controversial subject in the field for many years. Na+K+ ATPase activity has been variously reported to be increased, decreased, or unchanged in bipolar patients. In 1997, Looney et al conducted a meta-analysis of the available literature on erythrocyte Na+K+ ATPase activity in bipolar disorders and concluded that it is lower in bipolar patients (Looney et al, Depress. Anxiety, 5:53-65 (1997)). However, the question of exactly how the Na+K+ ATPase plays a role in bipolar disorders remains unanswered.
Lithium, an alkaline metal that has been used successfully for over fifty years to stabilize mood in bipolar disorders, has been shown to augment Na+K+ ATPase activity. Recently, the role of lithium in depolarizing the resting membrane potential of neurons has been analyzed (Thiruvengadam, J. Affect. Disord., 65:95-99 (2001); and Thiruvengadam, “Electro-biochemical coupling, excitability of neurons and bipolar disorder, Bipolar Disorder 3 (2001)). Hyperpolarization of membrane potential in leukocytes of bipolar patients and depolarization following the addition of lithium has been observed (El Mallakh et al, J. Affect. Disord., 41:33-37 (1996)). In addition, a significantly smaller increase in Na+K+ ATPase density after incubation for 72 hours in ethacrynate or lithium has been observed in cells of bipolar patients compared to cells of unaffected individuals (Wood et al, J. Affect. Disord., 21:199-206 (1991)).
El-Mallakh et al measured the transmembrane potential in leukocytes from hospitalized bipolar patients and observed that the transmembrane potential of the bipolar patients was hyperpolarized compared with normal controls and euthymic patients on lithium (El-Mallakh et al, J. Affect. Disord., 41:33-37 (1996)). However, Buss et al measured the membrane potentials of cultured lymphoblasts and concluded that there was no significant difference in membrane potentials among bipolar patients, their siblings and normal controls (Buss et al, Psychiatry Res. 59:197-201 (1996)).
In view of the previously studies on the possible involvement of the Na+K+ ATPase in bipolar disorders, one would not expect Na+K+ ATPase activity to serve as a reliable basis for diagnosing a bipolar disorder in an individual patient, because measurements of Na+K+ ATPase activity are highly variable. Similarly, one would not expect transmembrane potential to serve as a reliable basis for diagnosing a bipolar disorder in an individual patient, because measurements of transmembrane potential are highly variable.
Accordingly, despite the existence of treatments for bipolar disorders and unipolar disorder and recent advances in the psychiatric field, there remains a heretofore unmet need for clinical tests to augment the DSM-IV in diagnosing bipolar disorders and unipolar disorder.